[Python] Add transport_model override option to Solution constructor

This keyword argument allows the transport model specified in a CTI/XML file to
be overridden when creating the object. Can be used to speed up Solution
initialization by specifying 'transport_model=None', which is especially useful
for large mechanisms where transport initialization scales as species number
squared.

interfaces/cython/cantera/base.pyx

@@ -1,6 +1,6 @@
 cdef class _SolutionBase:
     def __cinit__(self, infile='', phaseid='', phases=(), origin=None,
-                  source=None):
+                  source=None, **kwargs):
         # Shallow copy of an existing Solution (for slicing support)
         cdef _SolutionBase other
         if origin is not None:

interfaces/cython/cantera/composite.pyx

@@ -11,6 +11,9 @@ class Solution(ThermoPhase, Kinetics, Transport):
     `Transport`.  It defines very few methods of its own, and is provided so
     that a single object can be used to compute thermodynamic, kinetic, and
     transport properties of a solution.
+
+    To skip initialization of the Transport object, pass the argument
+    `transport_model=None` to the `Solution` constructor.
     """
 
 class Interface(InterfacePhase, InterfaceKinetics):

interfaces/cython/cantera/transport.pyx

@@ -25,7 +25,13 @@ cdef class Transport(_SolutionBase):
     # The signature of this function causes warnings for Sphinx documentation
     def __init__(self, *args, **kwargs):
         if self.transport == NULL:
-            self.transport = newDefaultTransportMgr(self.thermo)
+            if 'transport_model' not in kwargs:
+                self.transport = newDefaultTransportMgr(self.thermo)
+            else:
+                model = kwargs['transport_model']
+                if not model:
+                    model = 'None'
+                self.transport = newTransportMgr(stringify(model), self.thermo)
         super().__init__(*args, **kwargs)
 
     property transport_model: